This invention relates to a method and an apparatus for detecting sounds due to the breakage of glass.
Although the sound of breaking glass is usually readily identifiable to the human ear, the physical make-up of the sound depends on such matters as the thickness and area of the glass, the manner in which it is mounted, the height above floor level and the acoustic properties of the enclosure in which the breakage takes place. These factors make the identification of the sound from its physical characteristics, difficult and uncertain.
Conventional glass break detectors respond to frequencies above say, 4.5 KHz and are liable to produce false alarms upon detection of other sounds occurring in the frequency range of interest, for example telephone bells and rattling keys.
In an attempt to reduce the occurrence of false alarms much fine tuning of the detector is necessary once it has been installed on site.
It is known that the sound of breaking glass comprises an initial low frequency sound of fairly large amplitude followed by higher frequency sound as the fragments of glass collide with one another and with the floor. U.S. Pat. No. 4,134,109 for example, proposes analysing the pattern of acoustic signals in terms of signal strength of the initial sound and frequency content of subsequent sounds to discriminate between the breaking of glass and other sounds. If the amplitude of the initial sounds exceeds a given threshold and the frequency content of the later sound fits a predetermined pattern, an alarm signal is produced.
The system of U.S. Pat. No. 4,134,109 includes a complicated signal analysis circuit which is costly. Another disadvantage is the inability to discriminate between repeated large amplitude sounds containing high frequency components. It can be triggered by any sufficiently loud sound irrespective of its frequency.